Matthew Stevens overcame Neil Robertson 6-5 in a Haikou World Open semi-final that will be remembered as much for a dreadful error by veteran Welsh referee Eirian Williams as the snooker fare on offer.

Stevens will face defending champion Mark Allen in Sunday's best-of-19 frames final after the Northern Irishman produced some of the best snooker of his career to usurp an in-form John Higgins, who did little or nothing wrong as a carefree Allen raced away with the match by winning the final four frames in a 6-2 victory.

Stevens compiled breaks of 52, 69, 40, 79, 55, 65, 60 and a nerveless 51 in a final frame that he dominated to reach his first ranking event final since he lost 9-7 to Robertson at the 2008 Bahrain Championship.

The rejuvenated Welshman continued his return to form after wins over Shaun Murphy and Judd Trump in earlier rounds on China's Hainan Island, but should really have won by a more comfortable margin against Robertson, whose knock of 107 in the eighth frame belied an indifferent performance by his standards as he could not shake off Stevens.

While Robertson will perhaps want to forget about the nature of his defeat in over four hours, Welsh referee Williams will need to reflect with regret upon his performance after he wrongly replaced the balls during the fourth frame from a Stevens foul despite the help of TV technology in the form of HawkEye.

Why he chose to argue the point that he was in the right in a deserted arena and ignore the large television screens deployed to assist match officials in such situations was a grim error in modern times.

It probably cost Stevens the fourth frame as he missed a black moments later that would not have been available to him if Williams had correctly replaced the balls.

Robertson pinched that frame to level at 2-2, but thankfully for Stevens it did not bury his hopes of winning the match.

"It wasn't the best match, both of us struggled a bit. I made most of the breaks, but threw away a couple of frames and I would have been kicking myself if I'd lost," said Stevens, who was forced to borrow two cues earlier in the week after his cue was delayed in arriving from the UK.

"How I lost the ninth frame I'll never know, but I was only 5-4 down and I felt I was cueing well. "

Robertson confessed he was never at his best in humid conditions. "Didn't play well today," he said. "Couldn't pot a ball using side - so damp, but did the best I could. Good luck to Matthew in the final."

Stevens displayed some real grit to recover from losing the ninth frame to fall 5-4 behind after compiling 65 only to miss a reasonably straightforward brown and a red that would left Robertson needing a snooker.

The Melbourne player forced a re-spotted black with a 57 and won it after Stevens had missed three further chances. It was not costly against Robertson, but Allen looks to be a different proposition as he found arguably the finest levels of his career in usurping Higgins in a contest dripping with class.

It was certainly better than the level of play he produced in throttling Stephen Lee 10-1 to win last year's final.

Antrim's Allen moved 2-1 ahead with a 119 and a 45 after Higgins had opened with a 90.

The Scotsman drew level at 2-2 courtesy of a 58, but was forced to sit and admire his opponent as Allen ran into the type of form that Higgins had himself aired in demolishing Ding Junhui 5-0 in Friday's quarter-finals.

Higgins failed to pot a ball as breaks of 41, 56, 81 and 76 kept alive Allen's hope of becoming the first player to retain a ranking event since Higgins lifted back-to-back Welsh Open titles in 2010 and 2011.

Stevens will chase his second ranking event title a decade after he lifted his first one - the 2003 UK Championship.